The present invention pertains to a switch with a key displaceable in the longitudinal direction into a switch cover against the force of a spring, which said key switches an electric switching bridge at the beginning of its displacement, and which adjusts a variable resistor depending on the length of the displacement path. Such switches are used in electric hand tools, especially hand drills.
A switch of this general type is described in German application DE-OS No. 28 38 934. The motor of the machine is turned on via the switching bridge. The motor speed can be adjusted via a variable resistor or potentiometer. Locking of the key is not provided in the switch according to DE-OS No. 28 38 934.
A switching bridge according to German application DE-OS No. 28 38 934 is of a complicated structure, because it requires a separate draw spring in addition to the spring returning the key. In addition, its opposite contact must be connected to the corresponding terminal via an electrical connection.
In German application DE-OS No. 28 38 934 a slide contact of the variable resistor is displaced with the key in the longitudinal direction on a resistor track. The adjustment path of the resistor is thus equal to the actual travel of the key. This dependence is unfavorable, because the key travel is set based on criteria other than the adjustment path of the slide contact. The switch according to German application DE-OS No. 28 38 934 does not permit the user to preset the resistance value and thus also the speed in the case of a drill, which should be reached upon maximum actuation of the key. The user desires such presetting, e.g., when he is working on a material on which a certain working speed of the machine should not be exceeded. A switch in which a control knob is arranged on the key, such that the knob controls the extent to which the key can be pressed is described in European patent specification No. 00 11 440.
Such a control knob on the face of the key can practically only be operated by switching the machine off and removing it from its working position. The control knob becomes a nuisance for the user when he has to keep the key depressed over a rather long time, as is necessary in most work procedures.